Delayed Sleep Phase Disorder (DSPD) is a phenomenon in which the internal circadian clock is set to a time relatively late compared to desired sleep timing. DSPD is characterized by difficulty both going to sleep at a desired bed time and waking up in the morning. DSPD is very common in adolescents as they have both biological and social changes that cause a delay (latening) of circadian timing. This results in an abnormally late bedtime. The delay in sleep timing, by itself, is not problematic. When wake time is fixed, however, as it is for most adolescents attending school, DSPD results in a curtailment of sleep that is associated with a variety of negative consequences including depression, substance abuse, fatigue, poor academic or work performance, poor socialization, increased risk-taking behavior, and an increased risk for the development of diabetes and obesity. The loss of sleep in adolescence is pervasive and often debilitating for both the teen and the family. Bright light phototherapy is often prescribed for treatment of DSPD. This treatment consists of 2-3 hours of bright light administered every day prior to desired wake time to advance the timing of the circadian clock to an earlier hour. This means that an adolescent who needs to wake up at 7AM for school would need instead to wake up at 4 or 5AM and sit in front of bright lights for 2-3 hours every day, a difficult if not impossible set of instructions to follow. Recent data from our laboratory has shown that a sequence of brief, millisecond flashes of light can change the timing of the circadian clock. Furthermore, these flashes of light can change circadian timing while the subject is asleep. That is, the flashes can be administered during sleep and the light will penetrate closed eyelids. When light is administered in such a manner, it does not disturb sleep. We aim to examine the novel hypothesis that a sequence of bright light flashes administered during sleep to adolescents with DSPD can change the timing of the circadian clock such that it will be set to an earlier hour and enable an earlier bedtime. We hypothesize that this will improve the symptoms of DSPD as well as improve overall mood and behavior. In order to examine these hypotheses, adolescents enrolled full-time in high school will be recruited to take part in a one-month, at-home study. During every night of this month, participants will be exposed to a sequence of flashes while they sleep. The quality of their sleep, as well as measurements of mood from both the adolescent's and the parent's perspectives, will be captured before, during, and at the end of the protocol. Results from this experiment could fundamentally change the manner in which DSPD is treated, significantly improving sleep in adolescents and simultaneously improving mood, academic performance, and family life. Results from this study will also be useful in understanding how light administration during sleep could be useful in the treatment of other circadian-based disorders, such as jet lag, shift work sleep disorder, and Advance Sleep Phase Disorder.